Coated abrasive



Novt s, 194e. H. F. KIRCHNER mL 2,410,506

COATED ABRASIVE 3 Sheets-Sheet lv1 Filed July 15, 1942 NOV. 5, 1946. Hp, K|RC||NER ETAL 2,410,506

COATED ABRAS IVE Filed July 15, 1942 3 sheets-sheet 2' Nov. 5, 1.946,' vH. P. KIRCHNER COATED ABRAS I VE Filed July 15, 1942 3 sheetsL-sheet s-ETP INVENTOR@ HENRY 1. KIRCHNER Mm ALM-Rr L. BALL ATTORNEY Patente 1946COATED ABRASIVE Henry P. Kirchner, Niagara Falls, and Albert L. Ball,Lewiston, N. Y., assig'nors to The (Jarborundum Company, Niagara Falls,N. Y., a corl poration of Delaware Application July 15, i942, Serial No.451,024

(C1. sl-issl 8 Claims.

This invention relates to abrasive coated belts or sheets, and has forone of its objects the provision of such coated abrasives in which theabrasive consists wholly or in part of diamond grit. Other objects willbecome apparent upon consideration of the following disclosure.

'Ihe belt or sheet of this invention is useful for grinding to shape,finishing, and polishing many different kinds of objects and materials,but nds particular utility in working on hard alloys, such as cementedtungsten carbide, boron carbide, and similar substances, or on hardferrous alloys such as special iron-tungsten alloys employed as dies andcutting tools. When employed as abelt, coated abrasive of this inventionmay be used for the purpose, among others, of sizing, finishing, andpolishing the interior working surface of v Wire, bar and tube drawingdies made of cemented hard carbide. These dies require nishing andpolishing after they are made by being molded and sintered. After acertain amount of use, such dies require a refinishing and repolishingto remove surface wear defects and to restore the mirror finishnecessary to their proper functioning. In addition, after such dies havebeen used for such time that their amount of wear exceeds a certaintolerance, their bore must be expanded to the next size and the dieagain finished and polished. Coated abrasives as provided by the presentinvention are suited for all these above-mentioned die finishing andpolishing operations, and their use greatly shortens the time andexpense required by the use of hitherto known abrasive materials incarrying out such working and finishing operations on dies, tools, etc.,made of hard alloy materials.

vThe present invention will be more readily understood by reference tothe accompanying'drawings, which are illustrative only and are not to beconstrued by way of limiting the present invention. In the drawings:

Figure 1 is an enlarged cross-section of a typical diamond coatedleather abrasive product, as taught -by the present invention.

Figure 2 is an enlarged cross-section of a diamond coated leatherabrasive product, the leather being reinforced by a layer of flexiblematerial united. to the back thereof.

Figure 3 is an enlarged cross-section of a diamond coated leatherabrasive belt having a relatively stii reinforcing layer joined thereto,said reinforcing layer having a quick-detachable splicen l Figure i is aTimes-section of a diamond coated Figure 6 is a cross-sectional view ofa draw-'- ing die as molded and sintered, and before finishing.

Figure 7 is a cross-sectional view of. a used drawingA die, andillustrates typical defects in the bore which must be removed. l

Figure 8 is a diagrammatic view of a belt grinding and polishing machinefor nishing the bores of dies, cylinders and the like. i

Figure 9 is an enlarged cross-section of a diamond coated leatherabrasivebelt having a narrow reinforcing member in the center of theback thereof.

Figure 10 is a cross-section through a die being finished by the beltshown in Figure 9.

Figure 11 is an enlarged cross-section of a diamond coated leather belthaving grain coated on the side portions of its face only, leaving thee. center portion devoid of grain. n

Figure 12 is a, cross-section through a die be ing finished by the beltshown in Figure 11.

Figure 13 is an enlarged cross-section of a diamond coated leatherabrasive belt having a longitudinal groove in the back face thereof.

Figure 14 is a crossmtection through a die being finished by the belt,fi'lown in Figure 13.

Figures 15-20, inclusive, are reproductions of charts produced by abrush surface analyzer, Figures 15, 1'?, and 19 being analyses ofsurfaces of cemented tungsten carbide abraded by diamond leather beltsof the present invention, and Figures 16, 18, and 20 being analyses ofsurfaces of cemented tungsten carbide abraded by belts composed of'backings of plied cotton tape with coatings of. diamond grit.

Figure 1 shows in enlarged cross-section an abrasive article typical ofthe present-invention. The article there shown is composed of a leatherbacking i, a coating 2 of a substance such as methyl cellulose, with orwithout a finely divided crevice filler such as clay, Whiting, ornon-diamond abrasive which prevents penetration of the making adhesivecoating 3, which may be a synthetic resin, into the leather, a layer ofabrasive granules 5, composed wholly or partly of diamond grit and asizing coat t of adhesive, which may likewise be a synthetic resin,applied `after the abrasive. grit, for the purpose of strengthening thebond between the grit and the base. Preferably the abrasive is appliedto the flesh side of "the leather, although it may be applied to thereverse side if desired, or to any cut surface of the leather. A coatingof a waterproofing substance is applied to the sides and back of theleather, thereby 'rendering the abrasivearticle impervious to moistureand able to be used with a liquid coolant.

A typical diamond-coated leather abrasive such as shown in Figure 1 ismade in the manner now set forth. It will, of course, be understood thatsuch method is set out merely for the purpose of illustration and thatnumerous-variations are possible, some of which will be given below.

A diamond coated leather belt is made by cutting'a piece of chrometanned leather 7 feet long, 1/4 inch wide and :54,4 inch thick.Obviously such leather should be as uniform as possible as far asdimensions, strength, resilience and grain 'are concerned. The fleshside of the cut piece is buffed on a 40 grit, bonded silicon carbidewheel to smooth it, after which the same side of the leather is sizecoated with a solution of methyl cellulose (5% by weight in- Water)and'hot dried for 30 minutes at 18o-200 F. The purpose of this coatingis to seal and stop the pores in the leather,

material, which may be, among others, that sold under the trade name"Viscol Triple Action, which is a vulcanized vegetable oil in solution.It is, of course, understood that other suitable waterproofing agentsmay be employed.

When the leather is waterproofed and when waterproof adhesives areemployed as the making or grain bonding coat and also the sizing coat,the coated abrasive oi the present invention may be used with a liquidcoolant. Such coolant allows the abrasive to take heavier cuts at higherspeeds than it can without its use, since the coolant keeps thetemperature of the coated abrasive below that at which it would beharmed, and also prevents the work piece from becoming locallyovcrheated and heat checked. In addition the coolant cleans detritusfrom the surface of the work piece, and prevents the scattherebypreventing undue penetration of the leather backing by the makingadhesive with the resulting undesirable embrittling of the backing. Insome instances a second coating .of size may be necessary to render thisface of the leather sufciently impervious to the making adhesive.

After the sizing coat or coats have dried, a making coat of liquidsynthetic resin is applied to the sized leather surface. A suitableresin, among others, is one sold under the trade name "Bakelite XR10190, which is an unmodified phenol-aldehyde resin which, when fullycured, sets to a firm, strong, and waterproof condition. This resin maybe mixed in the proportions of '75% by weight of the liquid resin to 25%by weight denatured ethyl alcohol. While the making coat is stillsticky, suitable grit is applied to it to form a uniform layer thereon.This grit may be wholly composed of diamonds, or maybe diamond grituniformly mixed with any desired proportion vof other abrasive. Theabrasive granule size may also be varied. Both factors depend on-thework to be done with the abrasive product. In one instance, a mixture of20% by weight of 220 grit diamonds and 80% by weight of 220 grit siliconcarbide may be used. The diamond 'and silicon carbide grit may also beused in ratios of 40/60, 80/20, etc., respectively.

The belt is now heatedvto dry the resin making coat. For the Bakeliteresin XR. 10190 such treatment consists of heating it at 160 F. for. 30

minutes, then `at 180 F. for 30 minutes, and

finally at 200 F. for 30 minutes. A sizing coat is then applied to thegrit coated side of the belt to strengthen the bond between grit andtering of dust cut from the work piece by the abrasive.

The making andsizing coats c'ure to a somewhat stiff condition. Toenable thebelt to assume thecontour of the guiding and driving pulleysas well'as to enable it to conform to the surfaces of curvedwork-pieces, it must be treated to cause uniform localized cracking ofthese coats without injuring the bond in locations where it is notcracked. 'One vmethod of accomplishing this is to gently flex the beltin the hands, and then to pull it over a large diameter pulley, such asa 6inch pulley, followed by flexing over'successively smaller pulleyssuch as 3-inch and then l-inch. The belt is then flexed over a 90 Woodencorner in directions along its length and then at 45 to its length ineach direction. The belt is also flexed cross-wise of its length bysupporting it at its longitudinal axis and bending down the edges.

It is essential that the belt joint be as strong and flexible aspossible. One manner of making a, satisfactory 'joint is to skive thebelt ends an'd slightly roughen them with abrasive paper. A cement suchas that sold under the trade name Sea Lion Waterproof Belt Cement, whichis a "cel1uloid type cement containing nitrocellulose and camphor in asolvent of methyl acetone, and which is made by the Chicago BeltingCompany, Chicago, Illinois, is applied to both ends in two coats, thefirst being rubbed in thoroughly and let dry for 20 .to 30 minutes. Thesecond coat is then applied, and the splice clooed at' once. The spliceis placed between plates and clamped under moderate pressure. The cementris allowed to set 10 minutes, the clamps are removed, and the'c'ementis allowed to dry in.

the air, after which the belt is ready for use.

Another manner of making a satisfactory joint is to skive the ends ofthe belt for a suitable distance. The beveled surfaces are then paintedwith a suitable adhesive bonding composition such as a solution of 10%by weight of a polyvinyl backing. Such size may be composed of a mixv F.held for 16 hours.

To render the belt waterproof itis coated on its back and sides with asuitable waterproofing acetal'resin sold under the trade name Alvar#l1-90 dissolved in 90% acetone. The skived ends are hot air dried for 5to 10 minutes, are then confronted, and are hot pressed at 300 F. for 2minutes on each side at a pressure of 500 to 600 pounds per square inch.

It will, of course, be understood that the above coating, sizing andcuring procedure is applicable to various leather-backed diamond coatedabrasives besides belts, and that numerous modifications of thisprocedure are possible. Thus the leather thickness may be varied, from aminlmum in cases where light cuts are taken and it is desired to havethe coated abrasive ilexible a layer 'i of suitable adhesive.

enough to conform to curves of short radius, to a maximum of leatherthickness available where straight surfaces or curves of long radius areto be cut, where the driving and guiding wheels are of large diameterand where coarse grit is employed for heavy cuts. Fairly thick'resilient belts are also useful for polishing operations as will beshown below. The width of theleather may likewise vary as desired inmaking belts and in the case of sheets may be any desired or availablevalue.

In general, the' stronger leather is the more satisfactory for suchdiamond abrasive belts, al-

though satisfactory belts are made with leather which has a tensilestrength of as vlow as about 2,000 pounds per square inch. Leathersranging up to a tensile strength of 11,670 pounds per square inch havebeen used satisfactorily. Resilience of the leather is another factortaken into consideration when belts are made. In general, the moreresilient leathers are chosen for the finer grit belts or sheets, since,as will be later explained, the excellence of leather as a backing forabrasive products of this kind appears to be due to its pronouncedresilience.

Experience has shown that for removal of line defects from cementedcarbide dies an abrasive of 220 grit size, in the ratio of 20 parts byweight of diamonds, 80 parts by weight of silicon carbide issatisfactory. To succeed the 220 grit belt, a 400 grit belt is generallyused with diamonds and silicon carbide in the ratio 40/60. This removesscratches left by the 220 grit belt. A 500 grit belt, with diamonds andsilicon carbide in the ratio 5/95 may be used to follow the 400 gritbelt, although usually it is necessary only to fol- 'low with the nalbelt 8'00 grit abrasive, dia-` mond/silicon carbide in the ratio 5/95.The 800 grit leather belt is eminently satisfactory for putting a iinalpolish and lustre on a die if the belt is a fairly thick soft backing,for example .045 inch or .050 inch thick. The ratios of diamond tosilicon carbide grit are obviously capable of being widely varied. Thefunction of the silicon carbide is primarily to -dilute the diamond gritand make possible different cutting rates at the same grit size andoverall grit concentration on the belt. uniform distribution of diamondson the belt than would ordinarily be possible without it.

Figure 2 illustrates a diamond-leather abrasive product similar to thatshown in Figure 1, with the exception that it is provided, on the face4opposite the abrasive, with a layer of flexible reinforcing material 8cemented to leather I by be composed, among other things, of metal foil.Its purpose is to strengthen the leather longitudinally and laterally,and also to prevent failure of the leather due to repeated exing. Ad-

It also allows the securing of a moreother at interface It. To preventlateral separation of the ends II and I2 of the belt during operation,one of the interfitting portions of hooks I0 and I3 is provided with agroove and the other with a rib, the rib and groove being such that theysnap by each other when the belt ends are disengaged. .The belt shown inFigure 3 .is run past the work piece in the direction shown by thearrow, so that end II of the belt is not sc uied up during working. Theleather is waterproofed on at least its exposed surfaces inthe Materialt may hesive 'I may be a hide glue, a silicate adhesive,

or if the product is to be waterproof, a heat hardenable resin such as aphenol-aldehyde condensation product. The other elements in Fig ure 2are the same as those in Figure 1.

Figure 3 illustrates a diamond-leather abrasive belt having a relativelystiff reinforcing member 8 secured to the back surface of the leather asby a layer of a suitable adhesive 9, which may be a heat hardenablesynthetic resin such as phenol-aldehyde condensation product. Leather Ihas coated on its upper or front face a layer of a sizing material suchas methyl cellulose, a layer of a resin making coat, a layer of diamonds'of suitable grit size, and a sand size coating over manner described inconnection with Figure 1.

Figure 4 shows a diamond-leather abrasive product having a reinforcingbacking I5 which may 'be metallicor non-metallic with a layer of eleather i6-secured thereto as by a layer of adhesive. Leather. I6, whichmay be in one piece or made up of several pieces cemented together, isshaped to a desired cross-section in order properly to t the contour ofthe work piece being abraded. A layer of diamonds Il is secured to Theleather facing may also be cupped, slotted,

or embossed to give the same effect.

Figure 6 shows a rough cored hard carbide drawing die or nib 22 asreceived from the sintering furnace. It willbe seen that the bore isdivided roughly into 5 portions, the first portion, 23, being known asthe'bell angle or radius; the second portion. 2t, being designated theentering angle; the third portion, 25, being known asthe approach angle;the fourth portion, 26, a right cylindrical bore being designated as thebearing;

and the last portion, 2l, being known as the back relief angle. The diebore must be finished to size and to such contour that the upperportions 23, 2t, and 25 become curved and blend smoothly into the lowercylindrical or bearing portion, and that the portion 2l also blends intothe bearing portion. The final finished bore of the die is indicated bythe dotted line 23 in Figure 2.

InFigure '7 there is shown ahard carbide.

drawing die 2e which, after extended use,` has developed grooves in thebore. Such grooves usually occur at the approach angle as shown in thefigure, although they may also develop in the bearing. Such grooves,which are shown greatly exaggerated as to depth in Figure 'I at 30, mustbe removed by refinishing and repolishing the bore of the die. Withshallow grooves it is possible to remove them and still have the bore ofa size within allowable tolerances. If such grooves' are deep, however,the bore must be expanded to the next usable size and reiinished andrepolished. The dotted line 3| indicates the contour of the bore afterbeing repolished.

refinished and Use of the apparatus diagrammatically shown in Figure 8is a convenient way in which the abrasiveproduct of the presentinvention may be utilized, when in the form of belts, to accomplish theabove-mentioned types of work on hard carbide dies or'dies of other hardmaterial. The apparatusconsists of a top guide and driving wheel 32 forabrasive belt 33, a floating bottom guide wheel 34,2. rotatable table 35driven by a means such as pinion 436, two top intermediate idler guiderolls 31, and two bottom intermediate idler guide rolls 38. Guide wheels34 may have its axle weighted to maintain and apply greater tension inthe belt and thus pressure of the belt against the work piece. All theseelements are supported on a suitable frame, not shown. Wheel 34 isadjustable in a vertical direction in order to accommodate belts ofdifferent lengths. Idler guide rolls 31 and 38 are adjustable verticallyand also to and from the line between the centers of wheels 32 and 34,Whereas wheels 31 are shown inside the belt and wheels 38 outside thebelt, either set may be employed inside or outside the belt, dependingon the contour of the bore being finished in die 22. Die 22 is held ontable 35 by a suitable chuck, not shown. Table 35 is rotated by means 36in synchronism with the movement of belt 33 through the die, and thusthe die is uniformly abraded in each peripheral-zone of its bore.Adjustment of table 35 vertically, suitable adjustment of guide rolls31-and 38, and a proper choice of weighting on wheel 34 make possiblethe attainment of desired belt pressure in any vertical zone of thebore.-

By `suitable cutting and gauging operations the rough cored die shown in-Figure 6 may be iinished entirely by use of diamond abrasive leatherbelts on the machine shown in Figure 8. The removal f grooves orscratches and repolishing of the die, as shown in Figure '7, aswell asthe openingup of badly worn dies to the next size, and refinishing andrepolishing the same, may likewise be carried out by use oi' diamondabrasive leather belts on the device in Figure 8.

As an example of the procedure followed in bringing the bore ,of therough cored nib shown in Figure 6 to the finished size and contour, the

-following steps are carried out. A leather belt having a mixture of 220grit abrasive thereon, said abrasive consisting of a mixture of diamondgrit and silicon carbide grit in the ratio of 20/80 is employed toremove a large amount of material to cause the bore to approach finishedshape. A leather belt with a mixture of 400 grit diamonds and 400 gritsilicon carbide in the ratio 40/60 is then employed to remove thescratches resulting from the 220 grit belt and to bring the `bore to a.partial mirror finish. As a final step a leather belt is used which isprovided with 800 grit abrasive, said abrasive consisting of 800 gritdiamonds and 1000 and finer grit. silicon carbide in the ratio /95, forthe purpose of vremoving the scratches left by the 400 grit and todevelop the final lustre, which is such that the finished surface is averitable mirror.

A similar procedure is followed in the opening of a die to a larger sizeor in removing vgrooves or scratches. .The choice of the grit size ofthe first belt employed depends upon the amount of stock to be removed.If it is small, a grit size of 'around 400 can first be used, followedby the 800 grit for finishing and polishing.

The following illustrates 'the -saving in time,

in the resizing 0f a warn die of cemented tungsten carbide 1 inch long,from 3A inch diameter to its next useful size of H inch diameter.Ordinary procedure for accomplishing this is as follows:

' A. The die is rebored with a diamond tipped tool at not-over .001 inchper cut with very slow feed; time, 8 to 12 hours.

B. The bore of the die is now lapped with 120 boron -carbide held byolive oil in the meshes of a wire braid clad linen core, the wire beingsecurely twisted on to a hard wood stick which is held by hand in therotating' die.

C. The bore of the die is lapped with 320 boron carbide as in B.

D. The die bore is lapped with #5 diamond dust in olive oil lin meshesof a wire clad linen core twisted on a steel mandrel.

E. The bore of the die is lapped with #6 diamond dust on a hardwoodstick mandrel.

F. Some dies are occasionally given a further finishing polish with #6diamond dust and olive oil held on a cloth.

The lapping steps (B-F incl.) require from 3 to 4 hours. The total timeis thus from 11 to'16 hours to enlarge a 1%, inch die to H inch, andobviously the time involved increases with the size of the die.

When diamond-coated leather rough cutting and polishing belts made in.accordance with the present invention are employed in the manner set outbelow for enlarging die of the same dimensions, the increase in diametercan be accomoverconventional practice, possible by using a plished inabout 12 minutes actual cutting time, while the polishing can be done inabout 4 or 5 minutes. belts or diamond-coated leather sheets" is meant acoated abrasive having a backing composed entirely or predominantly ofleather, abrasive grit being coated on` the leather and retained by anadhesive, the abrasive being composed wholly of diamond grit or partlyof diamond grit, and partly of another kind or kinds of abrasive grit.The term leather as here used means the general class of animal, fishand bird hides, regardless of kind or nature of treatment in theprocessing of the hide. The belts shown in Figures 9, 11, and 13, aremodifications of that shown in Figure 1. Such modifications allow thebelt faithfully to conform to curvedA work piece surfaces and enable theproper grinding pressures to be applied where desired. Except as notedthe waterproofing, adhesive, and abrasive coatings of these belts arethe same as those in Figure 1.

The belt illustrated in Figure 9 consists of a layer 40 of leather suchas has been before described as useful in such belts, with a layer ofdiamonds or a' mixture of diamonds with other abrasive adhered to itsworking face in the same manner as that described in connection withFigure 1. A reinforcing member 39, which may -be among other suitablematerials, a thin springy strip of steel tape, made into a belt by buttor lap silver soldering its ends, is shown placed in a groove in theback of leather 40. In one example, which is illustrative only, thestrip 39 was 11g inch wide, 0.005 inch thick, and was placed at thecenter of a belt A inch wide, Another layer 4I of leather or suitablematerial is secured tothe leather 40 and strip 39 by a suitableadhesive. One such 4adhesive is that previously described which is soldunder the trade-name Sea Lion Waterproof Belt Cement, which is acellulold" type cement containing nitrocellulose and cam- By the-termdiamond-coated leather f ber 60, or if it is thin enough, neitherleather member need be grooved, since the leather will deformsufficiently under the combining presssure to conform closely to andenclose strip 39.

The belt shown in Figure 9, when employed to finish a die bore, as shownin Figure l0, by reason of the reinforcing strip 39 exerts considerablepressure on the work piece at the center in a location opposite strip39. The diiculty, sometimes experienced with belts not having such strip33, of securing the desired grinding pressure at the center of the beltwhen grinding curved surfaces, such as the bore of die d2 shown inFigure 10, is thus overcome. At the same time the belt flexes laterallyand longitudinally quite readily, and thus conforms faithfully to thesurface being ground.

`In Figure l1 there is shown a belt having a leather backing d3 with anabrasive layer of diamonds or a mixture of diamonds with other abrasivecoated thereon as before described, with the exception that the centerportion d@ of the belt is coated by a waterproofing. coat only. 'I'husthe center portion of the belt consists of exible tanned waterproofedleather only, and the belt readily flexes laterally at the center, asshown in Figure 12, when grinding on an interior curved surface,allowing substantially all the abrasive coated' portions of the belt tocome into contact with the work piece and to exert substantially uniformgrinding pressure. As an alternative the center portion of the belt maybe coated in the same manner as the other portions of the Working faceexcept that the abrasive is omitted.

Figure 13 shows a belt employing a leather base 615 having an abrasivelayer thereon of diamonds or a mixture of diamonds with other abrasivecoated thereon as before described. The back of the leather is providedwith a slot or groove 436, whereby the belt may ex readily laterally.Obviously more than one groove may be employed, if desirable, to producethe same result. As shown in Figure 14, such belt conforms readily tothe surface of the bore oi a die i2 or the like. It has been found thatby proper choice of depth and width `of the groove i6 correlated with.the width and thickness of the leather belt backing and the diameter ofthe bore to be ground, the belt can be made to exert substantiallyuniform pressure throughout the extent of its width on the die bore andthus each abrasive grain cuts eiciently,

It hasbeen found that leather-backed waterproof sheets or belts possessgreat and unexpected advantages over backings such as cloth or othercellulosic material. These advantages, which may result from the greaterresilience of leather as compared with cloth and the like, reside in theability to secure much smoother surfaces on Work pieces ground with suchleather sheets or belts. In every grit size, belts with leather backingsyield surfaces which are smoother and more lustrous than those groundwith belts having plied tape backings of the same thickness as theleather used in the comparable leather belt.

The superior surfaces obtainable with leather belts as compared withthose produced by use of plied tape backed belts are strikingly shown inthe charts reproduced in Figures 15-20, inclusive. These charts areproduced by the brush y surface analyzer in which a tracer pointconnected to a crystal pick-'up isA traversed over the surface to beanalyzed. Current developed by vibrations of the tracer, suitablyamplified, op crates a recording pen, Which'marks a path on a graphmoving in a known chosen speed relationship with the traversing speed ofthe tracer. The recorded line on the chart thus is a reproductionof thesurface analyzed, with vertical and horizontal dimensions multiplied bychosen known factors. A

In the charts reproduced in Figs. 15-20, incl., horizontal distances onthe work piece are multiplied 80 times. Thus 10 inches of chart lengthrepresent the tracing of a 1/8 inch length of work piece surface. In thecharts each small verticalY division represents 10 micro-inches (amicro-inch being l millionth of an inch) as measured on the work piecesurface.

The charts reproduced in Figures 15-20, inclusive, were made from sixdifferent surfaces ground on the same piecebf cemented tungstencarbide.` The leather and the tape belts employed the same size grit,that is, the producing the surfaces analyzed in Figures 15 and '16,Figures 17 and 18, and Figures 19 and 20,were identical as to backingthickness, width, adhesives in both making and sizing coats, and gritsize and amount. The only difference was that in Figures 15, 17 and 19the backing was of leather and in Figures 16, 18 and 20 the backing wasof plied cotton tape. In the belts employed to grind the surfacesreproduced in Figures 15 and 16 the grit was employed in aratio of 20parts by weight of diamonds to 80 parts by weight of SiC, designated20/80. In the belts employed to grind the surfaces reproduced in Figures16 and 17 the grit was employed in a ratio of 40 parts by weight ofdiamonds to 60 parts by weight .of SiC, Ai0/6() Whereas, the beltsproducing the surfaces analyzed in Figures 19 and 20 employed 5 parts byWeight of diamonds and 95 parts by weight of SiC, 5/95. The use ofsilicon carbide or other grit less hard than diamondsisfor the purpose,among others, as above set out, of diluting the diamond grit, thusallowing its uniform distribution on the belt.

As is readily apparent by comparison of Figures 15, 17 and 19 WithFigures 16, '18 and 20, respectively, the surfaces obtained by use ofleatherbacked belts are much smoother and more uniform than thoseobtained with plied tape backed belts. In every instance the maxima andminima of the traced curves in Figures 15, 1'? and 19 are less thanthose in 16. 18 and 20 respectively, and the curves in Figures 15, 17and 19 are more regular and uniform. \The comparison between leather andtape backed belts becomes more striking as the grit size is decreased,as can be seen by comparing Figures 19 and 20, where 800 grit wasemployed.

. The results given by the brush analyzer are borne out visually. backedbelts `give surfaces which are more -lustrous and of higher specularquality than do 800 grit diamond leatheubacked belt is very 5 In allcases the leathern polishing of drawing dies and the like, where-extremeaccuracy and as near perfection of polish as possible are required forsatisfactory operation, and the better polished the die surface thebetter will be the product and the more economical the use of the die.

One reason for the superiority of leather over cloth and the like as abacking for waterproof diamond sheets and belts is its greaterresilience. This greater resilience allows the leather belt to conformfaithfully to the surface being ground and thus the grit to contact itevenly and unlformly. Repeated contacts of this sort between the workand the grit, especially if it is ne, smooths and polishes the work to amirrorlike finish. The canvas, being less resilient, does not conform tothe work surface so faithfully. Thus certain grits take deeper'cuts thanothers and continue to do so on repeated contacts, scratching thesurface even though the grit may be ne and preventing the attainment ofthe required mirror finish on the hard carbide and alloy dies, tools,

and the like.

An added advantage in the use of leather is that its resilient nature isfavorable toward a belt maintaining its cutting rate. This is a valuableattribute where skilled high wage labor is used in die finishingoperations. A diamond belt made on a cotton base was tested for droppingoff in cutting rate per unit interval of 12 ing comprised of a, layer ofleather cemented to a reinforcing layer of metal, a layer of abrasivegrain on the layer of leather, said grain comprising diamonds, and agrain bonding coat of an adhesive on the leather.

3. A coated abrasive belt comprising a backing consisting of a layer ofleather cemented to a time when grinding hard cemented carbide, and

compared to another diamond belt substantially the same except that thebacking was leather. Such advantage is clearly shown in the followingtable:

Per cent of initial cutting rate C L th ecentage otton ea er erence,Tx-me interval base belt base belt degree of superiority reinforcinglayer of metal by a layer of a cured phenol-aldehyde resin., a layer ofabrasive grain on thelayer of leather, said grain comprising diamonds,and a grain bonding coat of a cured phenol-aldehyde resin on theleather, the `leather being waterproofed at least on its exposedsurfaces.

4. A vflexible coated abrasive comprising a backing comprised of a layerof leather, the working face of the leather having uniformly distributedportions of greater height than the remainder of such face, a layer ofabrasive grain on such raised portions of the Working face of theleather, said grain comprising diamonds, and a grain bonding coat of acured resin on the raised portions of the working face of the leather.

5. A flexible coated abrasive belt comprising a backing of leather, alayer of abrasive grain on the leather, said grain comprising diamonds,and a grain bonding coat of an adhesive on the leather, said leatherbacking having incorporated therein a reinforcing strip of relativelystiff springy material, said. reinforcing strip being located asubstantial distance from the abrasive coated face of the leatherbacking.

6. A flexible coated abrasive beltl comprising a backing of leather, alayer of abrasive grain on the leather, said grain comprising diamonds,and

a grain bonding coat of an adhesive on the leather, said leather backinghaving at least one longitudinal groove in the back thereof atapproximately its center.

7. A flexible coated abrasive belt comprising a backing of leather, alayer of abrasive grain on the side portions of the working face of thebelt, the center of the belt being devoid of Vabrasive grain,.said graincomprising diamonds, and a grain bonding layer of an adhesive on theleather.

8. A coated abrasive waterproof belt comprising a backing consisting ofa layer of flexible resilient leather, said leather having a porestopping sizing coat on one surface and being waterprcofed on its othersurfaces, a coating of abrasive grain on the sized surface of theleather, said abrasive grain comprising at least a substantial amount ofdiamonds, and a grain bonding coat of a cured phenoll-aldehydeWaterproof resin on the sized surface of the leather.

HENRY P. KIRCHNER. ALBERT L. BALL.

